Digital radio broadcasting technology delivers digital audio and data services to mobile, portable, and fixed receivers. One type of digital radio broadcasting, referred to as In-Band On-Channel (IBOC) digital audio broadcasting (DAB), uses terrestrial transmitters in the existing Medium Frequency (MF) and Very High Frequency (VHF) radio bands. HD Radio™ technology, developed by iBiquity Digital Corporation, is one example of an IBOC implementation for digital radio broadcasting and reception.
The National Radio Systems Committee, a standard-setting organization sponsored by the National Association of Broadcasters and the Consumer Electronics Association, adopted an IBOC standard, designated NRSC-5, in September 2005. NRSC-5, the disclosure of which is incorporated herein by reference, sets forth the requirements for broadcasting digital audio and ancillary data over AM and FM broadcast channels. The standard and its reference documents contain detailed explanations of the RF/transmission subsystem and the transport and service multiplex subsystems. Copies of the standard can be obtained from the NRSC at http://www.nrscstandards.org/standards.asp. iBiquity's HD Radio technology is an implementation of the NRSC-5 IBOC standard. Further information regarding HD Radio technology can be found at www.hdradio.com and www.ibiquity.com.
Both AM and FM In-Band On-Channel (IBOC) hybrid broadcasting systems utilize a composite signal including an analog modulated carrier and a plurality of digitally modulated subcarriers. Program content (e.g., audio) can be redundantly transmitted on the analog modulated carrier and the digitally modulated subcarriers. The analog audio is delayed at the transmitter by a diversity delay. Using the hybrid mode, broadcasters may continue to transmit analog AM and FM simultaneously with higher-quality and more robust digital signals, allowing themselves and their listeners to convert from analog-to-digital radio while maintaining their current frequency allocations.
The digital signal is delayed in the receiver with respect to its analog counterpart such that time diversity can be used to mitigate the effects of short signal outages and provide an instant analog audio signal for fast tuning. Digital radios operating in a hybrid analog-digital mode incorporate a feature called “blend” which attempts to smoothly transition between outputting analog audio and digital audio after initial tuning, or whenever the digital audio quality crosses appropriate thresholds.
In the absence of the digital audio signal (for example, when the channel is initially tuned) the analog AM or FM backup audio signal is fed to the audio output. When the digital audio signal becomes available, the blend function smoothly attenuates and eventually replaces the analog backup signal with the digital audio signal while blending in the digital audio signal such that the transition preserves some continuity of the audio program. Similar blending occurs during channel outages which corrupt the digital signal. In this case the analog signal is gradually blended into the output audio signal by attenuating the digital signal such that the audio is fully blended to analog when the digital corruption appears at the audio output.
Blending will typically occur at the edge of digital coverage and at other locations within the coverage contour where the digital waveform has been corrupted. When a short outage does occur, as when traveling under a bridge in marginal signal conditions, the digital audio is replaced by an analog signal. Various blend algorithms have already been developed and implemented in commercial HD Radio receivers. The existing algorithms detect corrupted digital audio frames by performing cyclic redundancy checks (CRCs) on corresponding audio packets. The CRC results (1=pass, 0=fail) are filtered over roughly one second to provide a short-term Digital Signal Measure (DSM). This time constant is short enough to initiate corrective action when the digital signal is poor. The output audio is blended from digital to analog when the digital signal is corrupted, i.e., when the filtered CRC success rate (DSM) falls below a predetermined threshold. Conversely, the output audio is blended from analog to digital when the DSM exceeds a higher predetermined threshold.
Other signal-quality metrics, such as signal-to-noise ratio (SNR) or signal-to-noise ratio values (Cd/No), have been employed to reduce frequent blends between analog and digital audio when the coverage is intermittent, such as in mobile multipath fading and shadowing. However, this has been found to unnecessarily reduce the static digital coverage area by several radial miles, corresponding to a loss of several dB of signal strength. Existing algorithms have also attempted to suppress blending when other blends have recently occurred, using a timer and blend history but neglecting the importance of digital signal quality estimates.
Although it may seem reasonable to fill-in every digital audio gap, this is generally not desirable because corresponding analog and digital audio segments do not necessarily sound the same. Intentionally different audio processing on the analog and digital audio streams degrades the listener experience while blending. In addition, any time or level misalignment between the two streams further degrades the audio quality while blending between them. Perfect alignment is neither well-defined nor practical due to the different processing between the audio streams, resulting in frequency-dependent group delay and level differences.
Blending between audio streams can be physically accomplished by reducing the gain of one stream while complementarily increasing the gain of the other, using a ramp gain-change over about one second. Timing misalignment and group delay differences between the two streams can result in a degraded audio signal during the blend ramp. Therefore, it may be desirable to minimize blend events at the expense of some digital coverage when it is intermittent. Maintaining analog audio is generally preferred over allowing short intermittent digital audio bursts. In addition, field test results and listener feedback suggest that existing blending often occurs too frequently, is sometimes annoying, and can be overly complicated and difficult to configure.